1. Field of the Invention
The present invention relates to an electric motor drive device for a camera and, particularly, to an electric motor drive device which performs a starting action without fail.
2. Description of the Prior Art
Heretofore there have been proposed various kinds of electric motor drive devices for cameras which automatically perform wind-up actions such as a film winding action, a shutter winding action, and the like. Some of these conventional drive devices, which are arranged to be incorporated in the camera, are arranged as an external accessory to be used by mounting it on the camera prior to use. Among the conventional electric motor devices, one type has recently come to be widely used which permits an electric motor winding action of a camera by operating a button. Such button is provided on the drive device and is equivalent to a shutter release button or a release button, operating after the start of the camera and completion of a photographing operation.
Such approach has resulted from the increasing application of electronics to the control circuits of cameras and from the design of the starting action of a camera by an electrical releasing method using an electromagnet instead of a mechanical release mechanism. This arrangement permits operation with a soft touch, continuous photographing, external remote control, etc. and is advantageous for various kinds of photographing camera systems.
An electrical motor drive device, irrespective of whether it is incorporated in a camera or used as accessory thereto, permits control of starting action when timing information on the completion of a photographing operation is received as an electrical signal. This, in turn, permits electrical control over signal operation on the side of the camera and is highly advantageous for various applications of electronics in cameras.
For such applications, however, the following problem with the conventional devices has arisen. In order to form an input signal for a release operation member or the like of the camera, a switch is operated by a camera operator for photographing, irrespective of whether the switch is a mechanical switch or a solid switch using a conductive rubber. The camera operators, however, have different personal habits, peculiarities or inclinations. Some of them operate quickly and some tend to operate incompletely. The switch is thus operated in an infinite variety of ways.
For example, consider a case where an operator pushes a camera starting button 1 to operate a dual switch S as shown in FIG. 1 of the accompanying drawings. Then, even when contacts Sa and Sb are brought into contact with each other by the first stage stroke of a depressing operation on the button 1, there first takes place a chattering phenomenon which causes a transient signal to be formed corresponding to the speed of operatin on the button 1 and also to the surface conditions of the contacts Sa and Sb before a signal based on normal contact between the contacts Sa and Sb is produced. The same phenomenon also takes place when the contact Sb is brought into contact with another contact Sc by the second stage stroke of the button depressing operation. In this manner, when an operation switch of such a type is used, there are produced transient chattering signals not only in the depression movement but also in the button return movement of the button depressing operation.
Now, assume that a starting button of a camera or of an electric motor drive device is arranged such that, after a light measuring action of the camera is performed in response to a signal e1 obtained from contact made between the contacts Sa and Sb of the switch S and after the release of a shutter to perform a photographing operation in response to a signal e2 obtained from contact made by the contacts Sa, Sb and Sc of the switch S, a wind-up action is performed by an electric motor drive action taken in response to a signal e3 produced as shown in FIG. 2. The chattering takes place when the finger F of the operator moves upward. Under this chattering condition, the electric motor drive is rendered intermittent by the signal e3 or, if the vibration of the chattering is quick, no wind-up action would be performed at all and only a queer sound would be produced from a motor of the device. Conversely, if the vibration of the chattering is slow, the wind-up action would be effected only halfway. In any case, it has been impossible to expect a smooth wind-up action under such a condition. With a wind-up action effected halfway in this manner, an intermittent driving action causes a winding mechanism to vibrate. Then the vibration is transmitted to the finger F to accelerate the chattering and, thus, a satisfactory wind-up operation becomes hardly possible.
A second problem with the conventional electric motor drive devices also exists. This problem occurs when a control circuit which is provided for controlling the drive action of a motor is arranged to use a driving power source of the motor in common therewith. The power source voltage varies when a large driving current flows to the motor. Accordingly, this variation in the power source tends to cause the control circuit to operate erroneously. When a battery is used as a power source, limitation of available space forbids use of a battery of large capacity. Therefore, a great degree of power is consumed for driving the motor and this causes a decrease in the electromotive force of the battery. In addition to this, the voltage drop which takes place when the motor is driven makes it impossible to have a sufficient supply of power to the control circuit. Under such circumstances, there is a high probability that a variation in the battery voltage might be mistaken for an input signal. Under such a condition, therefore, an accurate control action of the control circuit may likely occur.
The present invention is directed to the elimination of these shortcomings of the conventional electric motor drive devices.